The cast of the BBC’s Jamaica Inn failed to deliver their lines clearly, the corporation’s head of drama has admitted after a quarter of the audience gave up on the drama.

Ratings for the Daphne du Maurier adaptation fell from 6.1 million for Easter Monday’s opening episode to 4.5 million for episode two.

Around 800 viewers complained about the “incoherent mumbling” of the cast – a record number for a sound issue – and 252 of those came after the second episode, despite the BBC promising to deal with the problem. Initially, the BBC blamed “issues with sound levels” on the master tape. The writer, Emma Frost, said there was a problem with transmission.

However, following a second night of complaints in which viewers said the sound had barely improved, the corporation appeared to backtrack.

Ben Stephenson, the head of drama commissioning, conceded: “I think actors being clear is one part of it.

“My understanding of the complaints about Jamaica Inn was more complex than that, so I think it’s probably not right to single out that, but clearly we want actors to speak clearly.

“Of course we want them to give brilliant performances and you’ve got to respect that, but if no one can understand what they’re saying, then there’s a problem.”

The BBC’s original suggestion that technical problems were to blame drew an angry response from sound mixers.

Ian Sands, vice-chairman of the sound branch for the London production division of broadcasting union Bectu, said: “If the actors are allowed, or encouraged, to mumble their lines then that is what is recorded. Low-level, mumbled lines are not a technical issue, they are an artistic issue. Many directors, and it may not be the case here, are very reluctant to tackle actors about their performance.”

Mr Sands said that modern flat-screen televisions exacerbate the problem because their speakers face the wall or the floor. He said programme-makers failed to understand that “the home listening environment is nothing like the high quality environment where the final mix takes place”.

Mr Sands added: “The final sound balance for transmission is signed off by the director and any adjustments and balance are done to the director’s requirements, whether they happen to be right or not.”

The director, Philippa Lowthorpe, declined to comment.

In an earlier interview, she spoke of her passion for Jamaica Inn and praised the script for being short on dialogue.

“A lot of the time it’s not about what the characters are saying, it’s about what they are not saying,” she said. “The dialogue in Emma’s script is spare, which makes it really cinematic, and gives room for the complexity of the relationships to come out between the characters in a more visceral and visual way.”

Viewers singled out Sean Harris, who played the evil uncle of Mary Yellan (Jessica Brown Findlay) for particular criticism.

“Sorry BBC, episode 2 was no better. It’s not sound quality that’s the problem, it’s diction. The actors, and it’s one in particular, just mumble and we cannot understand a word,” one viewer wrote on the BBC’s online messageboard.